The correct answer is option 3, Kanaganahalli. The statement in the question is referring to the only known inscribed stone portrait of Emperor Ashoka, which has been found at Kanaganahalli.

To understand the significance of this, it is important to note that Emperor Ashoka was an important Indian ruler who played a key role in the spread of Buddhism in ancient India. He is known for his rock and pillar edicts, which were inscriptions that conveyed his principles and teachings.

The inscribed stone portrait at Kanaganahalli is particularly significant because it provides a visual representation of Emperor Ashoka. It helps us to understand what he looked like and adds to our knowledge of his reign.

The other options mentioned in the question are Sanchi, Amaravati, and Ajanta. While all of these places are known for their historical and archaeological significance, they do not have a stone portrait of Emperor Ashoka. Hence, they are not the correct answer to the question.

In summary, the only inscribed stone portrait of Emperor Ashoka has been found at Kanaganahalli.